The present invention relates to a subsea installation for, for example, separation of hydrocarbons from a subsea well. The invention also relates to a method for separating liquid from gas from a subsea well stream by means of gravitational forces.
Reservoirs from which hydrocarbons are produced may be roughly divided into two types:
1. Gas/condensate reservoirs
2. Oil reservoirs
The well stream from a gas/condensate reservoir will mostly contain natural gas, but will also contain condensates (light hydrocarbons in liquid phase). In addition it may contain water, either in the form of condensed water which is in equilibrium with the gas under existing pressure and temperature, or produced water from the reservoir. There may, moreover, be chemicals added at the wellhead in order to avoid corrosion and gas hydrates.
The well stream from an oil reservoir mostly contains oil, but there will normally be a varying fraction of natural gas. In addition this well stream may also contain water and added chemicals such as corrosion inhibitors, scale inhibitors, etc.
Whether it is produced water or condensed water, the water is not a commercially exploitable resource in the well stream and may cause problems such as increased pressure loss, slugs, corrosion, scaling, emulsions and hydrates in the transport system.
When hydrocarbons are recovered from a subsea well, the existing reservoir pressure is used to transport the liquid and gas mixture from the reservoir up to the wellhead and through the transport pipeline to a receiver terminal or to a floating production unit (FPSU—Floating Production Surface Unit).
The part of the transport line that extends on the seabed will have natural undulations and the liquid fraction of the well stream will collect at the lowest points when the well stream velocity is sufficiently low. When sufficient liquid has collected, a slug may be formed which is driven by a high back pressure and pushed through the pipeline system. This slug is an undesirable event in a multiphase transport system and may lead to problems for the receiving system in the form of rapid pressure changes and the risk of liquid in undesirable parts of the receiving system. Where the slug reaches an onshore installation, large mechanical constructions (slug catchers) have to be built for damping these pressure changes and storing of the incoming volume of liquid. The drawback with this solution is that it requires a great deal of space.
During production to an FPSU, particularly when the unit is located in a deep-water maritime area, the slug will induce stress on the riser system, in addition to which the liquid column will increase the counter-pressure on the reservoir, thereby creating rapid pressure and volume flow changes which are difficult to handle on the floating unit. This problem is normally remedied by injecting gas into the bottom of the riser, thereby helping to lift the slug up through the riser. The disadvantage of this solution is that the pressure loss in the riser will be high on account of increased friction.
Another solution is to install one or more separator tanks on the seabed, where the gas fraction is separated from the liquid, and the liquid and the gas are passed up to the surface in separate risers. The liquid then has to be pumped up to a higher pressure. The disadvantage of this solution is the size of the separator tank which must have a large diameter in order to provide good separation. On account of high external or internal pressure, it is necessary to have thick-walled tanks, which makes them difficult to produce with today's technology. In addition the separator tanks often have complex insides requiring maintenance, which in turn means that they must be capable of being pulled up to the surface. This is time-consuming and costly. From the design point of view this means that the separator tanks on the seabed must consist of several mechanical constructions which can be connected by means of a remotely controlled installation system. If the device is installed in an area where fishing is conducted, the installation must also be capable of being trawled over, thereby also increasing the size and weight of the equipment.